This invention is in the field of plant molecular biology. More specifically, this invention pertains to isolated nucleic acid fragments encoding proteins with similarities to the barley Mlo protein and involved in corn (Zea mays) resistance to disease.
Southern or Maydis Leaf Blight is a serious disease of corn caused by Helminthosporium maydis Nisik. (Cochliobolus heterostrophus (Drechs)). While corn hybrids with normal cytoplasm are resistant to Race T of the pathogen, Race O shows no differential reaction between plants with different cytoplasms; however, sources of resistance are available (Shurtleff (1973) Compendium of Corn Diseases). Chlorotic lesion resistance to Race O was identified in an East African line of corn (Jeweus and Daniel-Kalio (1968) Plant Dis. Rep. 52:134-136). The resistant plants inhibit fungal sporulation and are distinguished by the presence of small chlorotic lesions. Significantly, the resistance is recessive (Smith and Hooker (1973) Crop Science 13:330-331). It was reported that disease is controlled by two linked recessive genes.
It is well established that resistance to many diseases in plants is mediated by the interaction of plant genes referred to as xe2x80x9cRxe2x80x9d genes with corresponding Avr genes expressed by the pathogen (Hammond-Kosack, K. E. and Jones, J. D. G. (1997) Ann Rev. Plant Phusiol Plant Mol. Bio. 48:1-39). This interaction leads to the activation of plant responses which in turn results in increased resistance to disease. This resistance is frequently mediated by the xe2x80x9chypersensitive responsexe2x80x9d, a localized cell death phenomenon that occurs in the areas of plant tissue invaded by the pathogen. This Avr/R interaction is race-specific; i.e., particular alleles of the plant R gene respond only to specific races of the pathogen which express a corresponding Avr gene (Flor, H. (1971) Ann. Rev. Phytopathol. 9:275-296).
Most of the R genes characterized to date are dominant; i.e., resistance is exhibited by plants carrying the appropriate R allele in either an heterozygous or homozygous state. Some R genes have been isolated by map-based cloning. Many homologs of R genes have also been identified by sequence similarity. Because of the rapid evolution of plant resistance to disease, homology of a cDNA or genomic DNA sequence to a known R gene from a different plant species does not allow one to predict the pathogen specificity of that particular R gene. No general methods for such prediction exist today.
Genes involved in resistance of plants to plant pathogens may be used to engineer disease resistance into plants normally sensitive to disease using several different approaches. Transgenic plants containing alleles of R genes demonstrate resistance to corresponding races of the pathogen (Wang, G. -L. et al. (1996) Mol. Plant-Microbe Interact. 9(9):850-855). The resistance genes may also be engineered to respond to non-native signals derived from the pathogen. In addition, pathogen-derived Avr genes may be expressed in a controlled manner in plants to strengthen the response to a pathogen. Genes further downstream from the R genes in the signal transduction pathways that transmit signals to the disease response effector genes may be engineered to directly respond to pathogen infection, thereby shortening the response pathway.
The process of the hypersensitive response to a pathogen, and the signaling networks involved, are poorly understood. In no case have all of the genes involved in transmitting the signal from the pathogen to the site of the initiation of the hypersensitive response been identified.
In a few cases, the functioning of a disease resistance mechanism different from the Avr/R interaction described above results in a recessive, rather than dominant pattern of resistance. One such example is the Mlo gene of barley, which conveys resistance to Erysiphe graminis f sp. hordei. The barley gene has been recently isolated by a positional cloning approach (Bueschges, R. et al. (1997) Cell 88:695-705). The dominant (sensitive) allele (Mlo) is thought to encode a protein involved in regulation of leaf cell death and in the onset of pathogen defense. The partial or complete inactivation of Mlo results in the priming of the disease-resistance response even in the absence of the pathogen, and leads to increased resistance to E. graminis. 
The available scientific data concerning Mlo-mediated disease resistance in barley points towards another approach to controlling disease: priming the pathogen response pathway by diminishing the effectiveness of negative regulation of the hypersensitive response. Appropriately engineered plants may show increased pathogen resistance at the expense of expressing some pathogen response-related genes even in the absence of pathogen. Sense or antisense inhibition or targeted gene disruption of Mlo and Mlo-related genes may have such an effect. Resistance to other pathogens may also be increased using this approach.
Mlo-related cDNA clones and DNA segments of genomic DNA, and their homologs and derivatives, may also be used as molecular probes to track inheritance of corresponding loci in genetic crosses, and thus facilitate the plant breeding process. Moreover, these DNA sequences may also be used as probes to isolate, identify and genetically map Mlo and other closely related disease resistance genes.
The instant invention relates to isolated nucleic acid fragments encoding corn homologs of the barley Mlo protein. Specifically, the instant invention relates to isolated nucleic acid fragments encoding plant proteins involved in the regulation of plant cell death response in corn which is associated with disease resistance. In addition, this invention relates to nucleic acid fragments that are complementary to nucleic acid fragments encoding a corn homolog of the barley Mlo protein.
An additional embodiment of the instant invention pertains to a polypeptide encoding all or a substantial portion of a corn Mlo protein.
In another embodiment, the instant invention relates to a chimeric gene that comprises a nucleic acid fragment encoding the corn Mlo or to a chimeric gene that comprises a nucleic acid fragment that is complementary to the nucleic acid fragment encoding the corn Mlo, operably linked to suitable regulatory sequences, wherein expression of the chimeric gene results in production of levels of a corn Mlo in transformed host cells that are altered (i.e., increased or decreased) from the levels produced in untransformed host cells.
In a further embodiment, the instant invention concerns a transformed host cell comprising in its genome a chimeric gene encoding a corn Mlo, operably linked to suitable regulatory sequences. Expression of the chimeric gene results in production of altered levels of the encoded protein in the transformed host cell. The transformed host cell can be of eukaryotic or prokaryotic origin, and include cells derived from higher plants and microorganisms. The invention also includes transformed plants that arise from transformed host cells of higher plants, and seeds derived from such transformed plants.
An additional embodiment of the instant invention concerns a method of altering the level of expression of a corn Mlo in a transformed host cell comprising: a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a corn Mlo; and b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of altered levels of corn Mlo in the transformed host cell.
An addition embodiment of the instant invention concerns a method for obtaining a nucleic acid fragment encoding all or a substantial portion of an amino acid sequence encoding a corn Mlo.
A further embodiment of the instant invention concerns a method for using the instant nucleic acid fragments and their homologs and derivatives as molecular probes to monitor inheritance of corresponding loci in genetic crosses, and thus to facilitate and accelerate plant breeding. Additionally, the instant nucleic acid fragments may be used as probes to isolate, identify and genetically map corn Mlos and other closely related disease resistance genes.